Bipolar disorder (BD) is a debilitating mood disorder that affects 1-2% of the population. Lithium is highly effective in BD, but the molecular mechanism that underlies the therapeutic response is unknown. Defining the cellular and molecular targets of lithium in the adult brain will be crucial to understanding the factors that contribute to this complex disorder. We have advanced the hypothesis that lithium acts on mammalian behavior and mood disorders through inhibition of glycogen synthase kinase-3 (GSK-3), a central regulator of multiple signaling pathways. The goals of the work proposed here are to establish how cellular and molecular responses to GSK-3 inhibition relate to the therapeutic response in BD, with a long-term goal to understand the molecular basis of affective disorders and to discover new therapeutic targets in BD and other affective disorders. This proposal describes experiments to define the signaling mechanisms that account for the effects of lithium on neural stem cell homeostasis and behavior. Our working hypothesis is that GSK-3 modulates both behavior and neurogenesis through interaction with Wnt signaling and oxygen sensing pathways. In addition, we hypothesize that GSK-3 is highly regulated by positive feedback circuits that can be targeted for the development of new therapeutic strategies in affective disorders. In aim 1, we will test whether Wnt signaling is required for the effects of lithium in either neurogenesis or behavior. We will also test whether neurogenesis is required for behavioral responses to lithium using a genetic strategy to target NSPCs in the adult hippocampus. Aim 2 addresses a new connection between oxygen sensing and the Wnt pathway, a major target of GSK-3 and a regulator of neurogenesis. We will explore the role of hypoxia inducible factor 1 (HIF-1) in the regulation of neurogenesis and lithium-sensitive behaviors and investigate the role of HIF-1 target genes in the response to lithium. In aim 3 we will explore a novel mechanism for the enhancement of GSK-3 inhibition by lithium and investigate approaches to perturb GSK-3 regulatory circuits as potentially new therapeutic targets in the treatment of BD and other affective disorders. These studies should provide a better understanding of the molecular and cellular mechanisms that underlie BD and the response to therapy and should lead to new approaches to the treatment of this common and devastating affective disorder.